Enterotoxigenic Escherichia coli, or ETEC, are a major cause of secretory diarrhea in man. ETEC belonging to the most frequently isolated serotypes and producing both the heat-stable and heat-labile enterotoxins also produce fimbriae which function in the pathogenesis of ETEC diarrhea. These fimbriae antigens are called colonization factors because these structures promote colonization of the intestinal mucosa by ETEC by an adherence mechanism. Two major colonization factor antigens (CFAs) are CFA/I which is produced by serotypes 015:H11, 025:H42, 063:H minus, 078:H11, 078:H12 and others, and CFA/II which is produced by serotypes 06:H16, 08:H9 and others. The ultimate goal of this project is to develop a vaccine for the prevention of ETEC diarrhea, which will consist of purified fimbrial CFAs. Anti-CFA/I and anti-CFA/II antibody responses have been demonstrated in field cases and in volunteers and in human breast milk. Also, passive protection of experimental animals with anti-CFA antibody has been demonstrated. Current work involved the development of an enzyme-linked immunosorbent assay (ELISA) for the quantitation of anti-CFA antibody according to immunoglobulin class (IgG, SIgA, etc) and this assay has been applied to serum, intestinal fluid and milk. Anti-CFA/I antibody responses have been shown in volunteers exposed to oral and parenteral doses of purified CFA/I (pure CFA/I is non-toxic). Further tests are to be performed on the response of volunteers to different doses of CFA/I administered at different times and considering the oral versus the parenteral route of administration. Related studies are concerned with the genetics of CFA/I and CFA/II production and the biochemistry of synthesis of these antigens.